This invention relates to control systems for steam turbines and more particularly to control systems for desuperheating the steam bypassing the turbine in a turbine bypass system.
In the bypass mode of steam turbine operation, a bypass path is provided around the turbine or a section thereof to divert any excess steam from the turbine that is not required to sustain its load. There are a number of expected advantages in this mode of operation, and although not widely used in this country in the past, there is an increasing interest on the part of electrical utility companies in operating their power generating facilities in this manner.
Contrasted with the more conventional mode of operation wherein the steam generator produces only that amount of steam required by the turbine, the bypass mode allows the boiler, or steam generator, to be operated independently of the turbine. The boiler may in fact be operated at essentially constant steam flow and pressure regardless of the load demand on the turbine. Any excess steam is simply fed to the bypass subsystem. Depending on the turbine and the section thereof being bypassed, the bypassed steam is routed to a condenser for recovery and recirculation as feed water to the boiler or to a reheater section of the boiler for subsequent passage to other sections of the turbine. In general, steam bypassed and that utilized by the turbine are combined in the same receiving unit, whether condenser or reheater.
It is apparent in considering this mode of operation, that the quantity of steam being bypassed varies considerably from time to time depending on turbine operation. Thus the bypass lines are expected to handle rather large quantities of steam when turbine load is low and to handle intermediate to zero amounts as load on the turbine is increased. Steam from the boiler passing to either the bypass subsystem or to the turbine, of course, contains a considerable store of energy in the form of superheat. Of the steam passing into the turbine, much of the superheat is expended as energy is extracted to drive the turbine. On the other hand, bypassed steam must also be desuperheated to substantially the same extent if overheating damage to the steam receiver (reheater or condenser) or subsequent sections of the turbine is to be avoided. The bypassed steam must be conditioned to have substantially the same impact on the receiving unit as steam received from the turbine.
An effective and practical method of desuperheating steam in a turbine bypass system is to spray liquid water into the steam to reduce its temperature. With the bypass system designed to accept up to 100% of the turbine flow, however, a problem results from the fact mentioned above, that the quantity of steam flowing in the bypass lines varies considerably with turbine operation. Although feedback control systems in which downstream steam temperature is determined and the water spray adjusted accordingly might be expected to provide satisfactory results, this is not the case. Feedback control, although effective over a narrow range of steam flow conditions, does not provide uniform control over the full range of bypass steam flow conditions. For example, a control loop making use of a conventional proportional-integral-derivative controller requires retuning with each sizable change in steam flow. This is simply not practical with the ever-changing steam flow which larger bypass systems are expected to contend with.
It is, therefore, a principal object of the present invention to provide an adaptive control system for a steam turbine bypass system which automatically adapts itself to changing steam flow conditions and which is capable of maintaining substantially uniform temperature control over the full range of anticipated steam flow conditions.
It is a further object of the invention to provide such adaptive control system with automatic protection against malfunctions in the desuperheating apparatus which tend to produce overheating or water induction.
Still further objects of the invention will become apparent from the ensuing description.